… to be able to play several parts at once ?
In the Virus Control I can choose up to 3 USB outs (shouldn’t there be 4 considering there are 4 parts?) so I assigned the first 3 parts to each of them.
Then in Renoise instrument settings I activated the 3 outs and assigned each to a different track.
What did I do wrong / what to do next ? Anytime I select a different part all MIDI channels use that part…
Solved 
Probably would be useful to mention how you solved it, for the benefit of any other Virus Ti users?
I would suspect the issue here was that only the main instance of the Virus Control plug-in was active. In order to address each part you have to create a VST alias of the Virus Control plug-in and assign it the relevant MIDI channel. Essentially you need a Renoise instrument to act as a proxy for each part you want to use.
This, while perfectly logical, can get a bit confusing and can bite you on the arse. Always check that you switch your actively selected part in Virus Control to match your currently selected alias’ MIDI channel. Many a time I’ve scratched my head wondering why, for example, the ludicrously named ‘curry’ overdrive isn’t applying to my new sexy lead. Only to run the sequence and discover, to my horror, that my bass now sounds like a Sumo wrestler jumping down a bag of shite! PANIC ENSUES as I try to undo the changes to part one before sheepishly switching to part two and making the changes…again.
Yep that was it 